sample.Sample doesn't have string format method

[ZhiQiang Fan]

which will cause only shows in logs:
Transform sample <ceilometer.sample.Sample object at 0x7f534ef378d0> from 0 transformer

we can see this in local environment or kibana, see: http://logstash.openstack.org/#eyJzZWFyY2giOiJwcm9qZWN0Olwib3BlbnN0YWNrL2NlaWxvbWV0ZXJcIiBBTkQgbWVzc2FnZTpcImNlaWxvbWV0ZXIuc2FtcGxlLlNhbXBsZVwiIiwiZmllbGRzIjpbXSwib2Zmc2V0IjowLCJ0aW1lZnJhbWUiOiI2MDQ4MDAiLCJncmFwaG1vZGUiOiJjb3VudCIsInRpbWUiOnsidXNlcl9pbnRlcnZhbCI6MH0sInN0YW1wIjoxNDE0NDE0MzkzNDE0LCJtb2RlIjoiIiwiYW5hbHl6ZV9maWVsZCI6IiJ9

in source code, where print logs about sample, are

* https://github.com/openstack/ceilometer/blob/master/ceilometer/pipeline.py#L272
* https://github.com/openstack/ceilometer/blob/master/ceilometer/pipeline.py#L293
* https://github.com/openstack/ceilometer/blob/master/ceilometer/transformer/conversions.py#L116
* https://github.com/openstack/ceilometer/blob/master/ceilometer/transformer/conversions.py#L137
* https://github.com/openstack/ceilometer/blob/master/ceilometer/transformer/conversions.py#L139

sample.Sample already has a method as_dict() to get formated string, but it is not nature for log, and developer must knew this detail and check carefully when they print log, I think we should better to provide a __repr__ method, which just do same thing as as_dict(), which will be much more convinent

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/131324

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/131324
Committed: https://git.openstack.org/cgit/openstack/ceilometer/commit/?id=21f926586b92e320001a6c42ed75ad283162d9d0
Submitter: Jenkins
Branch: master

commit 21f926586b92e320001a6c42ed75ad283162d9d0
Author: ZhiQiang Fan <email address hidden>
Date: Tue Oct 28 00:09:42 2014 +0800

    Add __repr__ method for sample.Sample

    sample.Sample has no string format method, which will cause useless
    message when print log.

    Change-Id: I8b9863a4ac6ee3373e18d36681fbfc4c89732111
    Closes-Bug: #1386187